U.S. patent number 9,382,359 [Application Number 14/420,313] was granted by the patent office on 2016-07-05 for reactor assembly and method for polymerization of olefins.
This patent grant is currently assigned to BOREALIS AG. The grantee listed for this patent is BOREALIS AG. Invention is credited to Mohammad Al-Haj Ali, Vasileios Kanellopoulos.
United States Patent |
9,382,359 |
Kanellopoulos , et
al. |
July 5, 2016 |
Reactor assembly and method for polymerization of olefins
Abstract
Reactor assembly for the production of polymers including a
fluidized bed reactor and method for operating the reactor
assembly.
Inventors: |
Kanellopoulos; Vasileios
(Espoo, FI), Al-Haj Ali; Mohammad (Helsinki,
FI) |
Applicant: |
Name |
City |
State |
Country |
Type |
BOREALIS AG |
Vienna |
N/A |
AT |
|
|
Assignee: |
BOREALIS AG (Vienna,
AT)
|
Family
ID: |
46963359 |
Appl.
No.: |
14/420,313 |
Filed: |
August 26, 2013 |
PCT
Filed: |
August 26, 2013 |
PCT No.: |
PCT/EP2013/002572 |
371(c)(1),(2),(4) Date: |
February 06, 2015 |
PCT
Pub. No.: |
WO2014/032794 |
PCT
Pub. Date: |
March 06, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150218295 A1 |
Aug 6, 2015 |
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Foreign Application Priority Data
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Aug 29, 2012 [EP] |
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12006133 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J
8/006 (20130101); C08F 210/16 (20130101); B01J
19/2465 (20130101); B01J 8/24 (20130101); B01J
8/1827 (20130101); B01J 8/388 (20130101); B01J
8/1872 (20130101); B01J 8/0055 (20130101); B01J
2219/00254 (20130101); B01J 2208/00938 (20130101); B01J
2208/00761 (20130101); B01J 2208/00274 (20130101); B01J
2208/00292 (20130101); B01J 2208/00893 (20130101); B01J
2208/00256 (20130101); B01J 2208/00672 (20130101); B01J
2208/00752 (20130101); B01J 2219/00247 (20130101) |
Current International
Class: |
C08F
2/00 (20060101); B01J 19/24 (20060101); B01J
8/38 (20060101); B01J 8/18 (20060101); C08F
210/16 (20060101); B01J 19/00 (20060101); B01J
8/00 (20060101); B01J 8/24 (20060101) |
Field of
Search: |
;526/59,67 ;422/131 |
References Cited
[Referenced By]
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Other References
International Preliminary Report on Patentability of International
Application No. PCT/EP2013/002572 dated Mar. 3, 2015. cited by
applicant .
"Gas Fluidization", Perry's Chemicals Engineers' Handbook,
McGraw-Hill, 2008, vol. 8, pp. 17-1 to 17-19. cited by applicant
.
Geldart, "Gas Fluidization Technology," J Wiley & Sons Ltd,
1986, pp. 156-169. cited by applicant .
Kirck-Othmer, "Gas Cleaning", Encyclopaedia of Chemical Technology,
vol. 10, 2nd Edition, 1966, pp. 340-342. cited by applicant .
Stolhandske, "Breaking your lumps: Crushers and how to select one",
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.
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.
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|
Primary Examiner: Cheung; William
Attorney, Agent or Firm: Roberts Mlotkowski Safran &
Cole, P.C.
Claims
The invention claimed is:
1. A method for polymerizing olefins in a fluidized bed reactor,
wherein a fluidized bed is formed by polymer particles in an
upwards rising fluidization gas, said upwards rising fluidization
gas has a superficial velocity in a middle zone of from 0.05 to 0.8
m/s, said method comprising the steps of: (i) withdrawing a
fluidization gas stream via an outlet from said fluidized bed
reactor at a height of more than 90% of the total height of said
fluidized bed reactor; (ii) separating polymer particles from said
fluidization gas stream to produce an overhead stream and a solid
recycling stream; (i) branching off from said solid recycling
stream a stream to downstream process stages; (iv) directing said
stream to downstream process stages to downstream process stages;
and (v) recycling the solid recycling stream into said fluidized
bed reactor; wherein said fluidized bed reactor comprises a bottom
zone, said middle zone and an upper zone, an equivalent
cross-sectional diameter of the bottom zone being monotonically
increasing with respect to the flow direction of the fluidization
gas through the fluidized bed reactor and an equivalent
cross-sectional diameter of the upper zone being monotonically
decreasing with respect to the flow direction of the fluidization
gas through the fluidized bed reactor; and wherein there is an
unobstructed passageway in the direction of flow of the
fluidization gas through the fluidized bed reactor from the bottom
zone to the upper zone; and the reactor further comprises an outlet
for the polymer whereby the polymer stream withdrawn from the
fluidized bed reactor via outlet of the polymer and the solid
recycling stream branched off to the output of the polymer are
combined for product recovery; and the reactor further comprising
gas/solid separation means, a flow through device, a solid
recycling line, a solid recycling inlet, a gas circulation line, an
inlet for fluidization gas located in the bottom zone and a
controller, the method further comprising the steps of: a)
measuring the mean particle size and/or the particle size
distribution and/or the concentration of all solids of an
fluidization gas stream from the fluidized bed reactor; b)
analyzing the operation conditions in the fluidized bed reactor; c)
sending the data obtained in steps a) and b) to a controller; d)
processing the data by the controller; and e) adjusting the flow
through device by the controller; whereby the flow through device
varies the solid recycling stream via solid recycling line back to
the fluidized bed reactor.
2. The method according to claim 1, wherein the fluidized bed
reactor further comprises a flow through device, the flow through
device branches off from the solid recycling stream entering the
flow through device a stream to downstream process stages directed
to down stream process stages through the line to downstream
process stages.
3. The method according to claim 1, wherein the fluidized bed
reactor further comprises one or more outlets for fluidization gas
streams located in the upper zone.
4. The method according to claim 1, wherein said fluidized bed
reactor comprises no gas distribution grid and/or plates.
5. The method according to claim 1, wherein the gas
solids/separation means are cyclones.
6. The method according to claim 1, wherein the outlet of the
polymer is located in the middle zone.
7. Method according to claim 1, wherein step b) comprises the
measurement of fluidization velocity, u.sub.f.
8. Method according to claim 1, furthermore comprising the steps
of: dd) predicting the mean particle size and/or the particle size
distribution of the fluidization gas stream using the data obtained
in step b); de) comparing the measured and the predicted mean
particle size and/or the particle size distribution of the
fluidization gas stream of steps a) and dd).
9. Method according to claim 1, whereby the solid recycling stream
is increased when the content of fines deduced from the measured
particle size distribution of step a) is larger than a
predetermined set point for the content of fines.
10. Method according to claim 1, wherein the flow through device
varies the solid recycling stream back to the fluidized bed reactor
and/or allows a stream to downstream process stages.
11. Method according to claim 1, wherein said fluidization gas is
upwards rising fluidization gas and said upwards rising
fluidization gas has a superficial velocity in the middle zone of
from 0.05 to 0.8 m/s.
12. A reactor assembly for the production of polymers including a
fluidized bed reactor comprising a bottom zone, a middle zone and
an upper zone, one or more outlets for fluidization gas streams
located in the upper zone, gas/solid separation means, a flow
through device, a solid recycling line, a solid recycling inlet, a
gas circulation line, an inlet for fluidization gas located in the
bottom zone; the outlet for the fluidization gas stream being
coupled with the fluidized bed reactor via a gas/solid separation
means, the gas circulation line and the inlet and via the solid
recycling line, the gas/solid separation means and the solid
recycling inlet; an equivalent cross-sectional diameter of the
bottom zone being strictly monotonically increasing with respect to
a flow direction of the fluidization gas through the fluidized bed
reactor; and wherein there is an unobstructed passageway in the
direction of flow of the fluidization gas through the fluidized bed
reactor from the bottom zone to the upper zone, characterized in
that the solid recycling line includes the flow through device,
wherein the flow through device allows for varying the amount of a
stream of particles, gas or fluid or mixtures thereof flowing
through the device whereby the variation occurs by adjusting the
flow through device and the flow through device lets pass 0 to 100%
of a stream in a certain direction and the flow through device
allows for passing the rest 100 to 0% of the stream in at least one
additional direction, and the gas/solids separation means are
cyclones.
13. The reactor assembly according to claim 12, wherein said
fluidized bed reactor comprises no gas distribution grid and/or
plates.
Description
This application is a U.S. National Stage Application under 35
U.S.C. Section 371 of International Application No.
PCT/EP2013/002572 filed on Aug. 26, 2013, and claims benefit to
European Patent Application No. 120061338.8 filed on Aug. 29,
2012.
The invention relates to a fluidized-bed reactor assembly for the
polymerisation of olefinic monomer(s), and to multi reactor
assemblies comprising at least one fluidized bed reactor.
Furthermore, the invention relates to a method of operating and use
of such a fluidized-bed reactor assembly.
BACKGROUND
Gas phase reactors are commonly used for the polymerization of
olefins such as ethylene and propylene as they allow relative high
flexibility in polymer design and the use of various catalyst
systems. A common gas phase reactor variant is the fluidized bed
reactor. In polyolefin production, olefins are polymerized in the
presence of a polymerization catalyst in an upwards moving gas
stream. The fluidization gas is removed from the top of the
reactor, cooled in a cooler, typically a heat exchanger,
re-pressured and fed back into the bottom part of the reactor. The
reactor typically contains a fluidized bed comprising the growing
polymer particles containing the active catalyst located above a
distribution plate separating the bottom and the middle zone of the
reactor. The velocity of the fluidization gas is adjusted such that
a quasi-stationary situation is maintained, i.e. the bed is
maintained at fluidized conditions. In such a quasi-stationary
situation, the gas and particle flows are highly dynamic. The
required gas velocity mainly depends on the particle
characteristics and is well predictable within a certain scale
range. Care has to be taken that the gas stream does not discharge
too much polymeric material from the reactor. This is usually
accomplished by a so called disengagement zone. This part in the
upper zone of the reactor is characterized by a diameter increase,
reducing the gas velocity. Thereby the particles that are carried
over from the bed with the fluidization gas for the most part
settle back to the bed. Yet another fundamental problem with
traditional fluidized bed reactors are the limitations as to the
cooling capacity and entrainment due to the formation of huge
bubbles. It should be mentioned that the presence of bubbles as
such is desirable, since mixing is intensified thereby. However,
bubble size should be much smaller than the diameter of the
reactor. Increasing the bed level in conventional fluidized bed
reactors for increasing the space-time yield leads to an increase
of the bubble size and to an unwanted entrainment of material from
the reactor. In conventional reactors there are no means for
breaking up the bubbles.
Various modified gas phase reactor designs have been proposed. For
example, WO-A-01/87989 has proposed a fluidized bed reactor without
a distribution plate and an asymmetric supply of the reaction
components to the reaction chamber.
Dual reactor assemblies comprising two reactors are also known. WO
97/04015 discloses two coupled vertical cylindrical reactors, the
first reactor being operated under fast fluidization conditions.
The first reactor having a frustoconical bottom zone and a
hemispherical upper zone is coupled with the second reactor being a
settled bed reactor. The operation under fast fluidization
conditions is done in a reactor having a ratio of length/equivalent
cross-sectional diameter of about 5 or more.
WO-A-01/79306 discloses a gas phase reactor assembly comprising a
reactor including a distribution grid coupled with a cyclone
separating solids and gaseous material. The separated solids are
recycled back to the reactor.
WO-A-2009/080660 reports the use of a gas phase reactor assembly as
described in WO-A-97/04015 comprising two interconnected reactors
and a separation unit, the first reactor being a so called riser
and the second reactor being a so called downcomer. The first
reactor is operated under fast fluidization conditions.
However, the fluidized bed reactors and the dual reactor assemblies
comprising a fluidized bed reactor described in the prior art still
have several disadvantages.
A first problem concerns the plugging of the underside of the
distribution plates due to entrainment of fines carried over with
the circulation gas. This effect lowers operational stability and
stability of the quality of the polymer. This problem partially can
be overcome by lower fluidization gas velocity. However, a
relatively low fluidization gas velocity limits the production rate
and can lead to the formation of sheets, chunks and lumps in the
production of polyolefins. This conflict of aims usually has been
countered by the incorporation of a disengagement zone. However,
disengagement zones again limit the production rate of a gas phase
reactor of fixed size, as there is the need for additional top
space above the top level of the fluidized bed during operation. In
industrial dimensions, the volume of the disengagement zone often
amounts to more than 40% of the total volume of the reactor and
insofar requires the construction of unnecessary huge reactors.
A second problem concerns the bubbling. Conventional fluidized bed
reactors typically operate in a bubbling regime. A part of the
fluidization gas passes the bed in the emulsion phase where the gas
and the solids are in contact with each other. The remaining part
of the fluidization gas passes the bed in the form of bubbles. The
velocity of the gas in the bubbles is higher than the velocity of
the gas in the emulsion phase. Further, the mass and heat transfer
between the emulsion phase and the bubbles is limited, especially
for large bubbles having a high ratio of volume to surface area.
Despite the fact that the bubbles positively contribute to powder
mixing, formation of too large bubbles is undesired because the gas
passing through the bed in the form of bubbles does not contribute
to the heat removal from the bed in the same way as the gas in the
emulsion phase and the volume occupied by the bubbles does not
contribute to the polymerization reaction.
Yet a further problem concerns the entrainment of solids containing
fines when removing the fluidization gas from the top of the
reactor. Especially when operating the fluidized bed reactor with
the bed level close to the roof of the reactor significant solid
entrainment occurs. However, the presence of solids in the
fluidization gas negatively affects the downstream units like
compressors, heat exchangers, etc. Therefore, means are used to
separate solids from the fluidization gas like for instance
cyclones. Cyclones operate by taking advantage of the higher mass
of the solids compared to the gas. Accordingly separation
efficiency of the cyclone deteriorates with a decreasing mass of
the solids. In other words increased amount of fines, i.e. small
solid particles with a small mass, entrained from the reactor
deteriorate cyclone efficiency as they are less efficiently removed
by the cyclone than the larger fraction of the entrained
solids.
Thus there is still the need for improved reactor design and
operation. The present invention aims to overcome the disadvantages
of the reactor designs known in the prior art and particularly aims
to avoid the segregation of fines at a high production rate. The
present invention further aims to increase the efficiency of
separating solids from gas. The present invention further aims at
avoiding low productivity zones in the reactor. Moreover, the
present invention concerns the provision of a reactor, allowing
high operational stability and at the same time production of
polymer having highest quality.
SUMMARY OF THE INVENTION
The present invention is based on the finding that these problems
can be overcome by a fluidized bed reactor assembly allowing for
variation of the amount of solids containing fines being recycled
to the fluidized bed reactor and/or varying the operation condition
of the reactor.
The present invention insofar provides a reactor assembly for the
production of polymers including a fluidized bed reactor (1)
comprising a bottom zone (5), a middle zone (6) and an upper zone
(7), one or more outlets (9) for fluidization gas streams (34)
located in the upper zone (7), gas/solid separation means (2), a
flow through device (29), a solid recycling line (35), a solid
recycling inlet (37), a gas circulation line (38), an inlet (8) for
fluidization gas located in the bottom zone (5);
the outlet (9) for the fluidization gas stream (34) being coupled
with the fluidized bed reactor (1) via gas/solid separation means
(2), gas circulation line (38) and inlet (8) and via solid
recycling line (35), gas/solid separation means (2) and solid
recycling inlet (37);
the equivalent cross-sectional diameter of the bottom zone (5)
being monotonically increasing with respect to the flow direction
of the fluidization gas through the fluidized bed reactor;
the equivalent cross-sectional diameter of the upper zone (7) being
monotonically decreasing with respect to the flow direction of the
fluidization gas through the fluidized bed reactor; and
wherein there is an unobstructed passageway in the direction of
flow of the fluidization gas through the fluidized bed reactor from
the bottom zone (5) to the upper zone (7),
characterized in that the solid recycling line (35) includes the
flow through device (29).
The present invention further provides a
method for operating a reactor assembly for the production of
polymers including a fluidized bed reactor comprising a bottom zone
(5), a middle zone (6) and an upper zone (7), one or more outlets
(9) for fluidization gas streams (34) located in the upper zone
(7), gas/solid separation means (2), a flow through device (29), a
solid recycling line (35), a solid recycling inlet (37), a gas
circulation line (38), an inlet (8) for fluidization gas located in
the bottom zone (5) and a controller (31), the method comprising
the steps of: a) measuring the mean particle size and/or the
particle size distribution and/or the concentration of all solids
of an fluidization gas stream (34) from the fluidized bed reactor
(1); b) analyzing the operating conditions in the fluidized bed
reactor (1); c) sending the data (30) obtained in steps a) and b)
to a controller (31); d) processing the data (30) by the controller
(31); and e) adjusting the flow through device (29) by the
controller (31);
whereby the flow through device (29) varies the solid recycling
stream (36) via solid recycling line (35) back to the fluidized bed
reactor (1).
Usually, the mean particle size and/or the particle size
distribution and/or the concentration of all solids of the
fluidization gas stream (34) is determined prior to the gas/solid
separation means.
The present invention further is directed to a
method for polymerizing olefins in a fluidized bed reactor (1),
wherein the fluidized bed is formed by polymer particles in an
upwards rising fluidization gas said upwards rising fluidization
gas has a superficial velocity in the middle zone (6) of from 0.05
to 0.8 m/s, said method comprising the steps of:
(i) withdrawing a fluidization gas stream (34) via outlet (9) from
said fluidized bed reactor (1) at a height of more than 90% of the
total height of said fluidized bed reactor (1);
(ii) separating the polymer particles from said fluidization gas
stream (34) to produce an overhead stream (42) and a solid
recycling stream (36);
(iii) branching off from said solid recycling stream (36) a stream
to downstream process stages (40);
(iv) directing said stream to downstream process stages (40) to
downstream process stages; and
(v) recycling the solid recycling stream (36) into said fluidized
bed reactor (1);
wherein said fluidized bed reactor (1) comprises a bottom zone (5),
a middle zone (6) and an upper zone (7), the equivalent
cross-sectional diameter of the bottom zone (5) being monotonically
increasing with respect to the flow direction of the fluidization
gas through the fluidized bed reactor and the equivalent
cross-sectional diameter of the upper zone (7) being monotonically
decreasing with respect to the flow direction of the fluidization
gas through the fluidized bed reactor; and wherein there is an
unobstructed passageway in the direction of flow of the
fluidization gas through the fluidized bed reactor from the bottom
zone (5) to the upper zone (7).
Preferably, the fluidized bed reactor is part of the reactor
assembly according to the present invention.
The present invention further is directed to the use of a
controller (31), a flow through device (29) and a solid recycling
line (35) in a reactor assembly for the production of polymers
including a fluidized bed reactor (1) comprising a bottom zone (5),
a middle zone (6) and an upper zone (7), one or more outlets (9)
for fluidization gas streams (34) located in the upper zone (7),
gas/solid separation means (2), a solid recycling line (35), a
solid recycling inlet (37), a gas circulation line (38), an inlet
(8) for fluidization gas located in the bottom zone (5) and an
outlet for the polymer (14) for minimizing the mass fraction of
fines with respect to the solids contained in the fluidization gas
streams (34).
The present invention further is directed to the use of a
controller (31), a flow through device (29) and a solid recycling
line (35) in a reactor assembly for the production of polymers for
minimizing the amount of fines produced by the fluidized bed
reactor (1), the reactor assembly including a fluidized bed reactor
comprising a bottom zone (5), a middle zone (6) and an upper zone
(7), one or more outlets (9) for fluidization gas streams (34)
located in the upper zone (7), gas/solid separation means (2), a
flow through device (29), a solid recycling line (35), a solid
recycling inlet (37), a gas circulation line (38), an inlet (8) for
fluidization gas located in the bottom zone (5) and a controller
(31).
The description of the method according to the present invention
applies to all embodiments of the invention.
The reactor assembly preferably further comprises a controller
(31).
DETAILED DESCRIPTION OF THE INVENTION
Definitions
An overview of different types of fluidization and different
fluidization regimes is given, for instance, in section 17 of
Perry's Chemical Engineers' Handbook, vol. 8 (McGraw-Hill, 2008).
FIG. 17-3 in Perry's shows that conventional bubbling fluidized
beds typically operate at superficial gas velocities between the
minimum fluidization velocity and the terminal velocity. The
turbulent beds operate at a gas velocity being close to the
terminal velocity. The transport reactors and circulating beds
operate at gas velocities significantly higher than the terminal
velocity. Bubbling, turbulent and fast fluidized beds are clearly
distinguishable and they are explained in more detail in Perry's,
on pages 17-9 to 17-11 incorporated by reference herewith.
Calculation of minimum fluidization velocity and transport velocity
is further discussed in Geldart. Gas Fluidization Technology, page
155, et seqq, J Wiley & Sons Ltd, 1986. This document is
incorporated by reference.
Fluidized bed reactors are well known in the prior art. In
fluidized bed reactors the fluidization gas is passed through the
fluidized bed within a certain superficial velocity. The
superficial velocity of the fluidization gas must be higher than
the fluidization velocity of the particles contained in the
fluidized bed as otherwise no fluidization would occur. However,
the superficial velocity should be substantially lower than the
onset velocity of pneumatic transport, as otherwise the whole bed
would be entrained with the fluidization gas. Reactors operating in
transport regime exist. Such operation is commonly referred to as
fast fluidization or fast fluidized beds. An overview is given, for
instance, in Perry's, pages 17-1 to 17-12, or M Pell, Gas
Fluidization (Elsevier, 1990), pages 1 to 18 and in Geldart, Gas
Fluidization Technology, J Wiley & Sons Ltd, 1986.
Solids and fines according to the present invention are both
particles. In particular, solids and fines according to the present
invention are polymer particles. Furthermore, the term solids as
used in the present application comprises the fines. According to
the present invention fines are defined as solids having a mean
particle size and/or particle size distribution (PSD) of less than
a defined threshold value, i.e, below that threshold value solids
are considered as fines. Where the threshold value is set depends
on the polymer grade as well as on the density of the polymer.
However, typically solids with a particle size (d.sub.p) of less
than 100 .mu.m, preferably, less than 80 .mu.m, more preferably
less than 50 .mu.m are defined as fines.
Particle size distribution may be characterized, by indicating,
both, the median particle size and the span of the particle size
distribution. The span is usually defined as
(d.sub.p,90-d.sub.p,10)/d.sub.p,50, where d.sub.p,90 is the
particle size for which 90% by the weight of the particles have a
diameter which is smaller than d.sub.p,90; d.sub.p,10 is the
particle size for which 10% by the weight of the particles have a
diameter which is smaller than d.sub.p,10; and d.sub.p,50 is the
median particle size for which 50% by the weight of the particles
have a diameter which is smaller than d.sub.p,50.
The gas/solid separation means (2) allow separation of gas and
solids. In the simplest embodiment this can be a vessel where the
solids, particularly polymer particles settle by gravity. Usually
the means for gas/solids separation comprise at least one gas/solid
separation unit which is preferably a cyclone. A cyclone in its
simplest form is a container in which a rotating flow is
established. Cyclone design is well described in the literature.
Particularly suitable cyclones are described in documents
Kirk-Othmer, Encyclopaedia of Chemical Technology, 2.sup.nd edition
(1966), Volume 10, pages 340-342 being incorporated by reference
herewith. The gas/solid separation means usually contains four
gas/solid sepratation units or less.
The solid filter means (41) also separates solids from gas. The
solid filter means is optionally present in the reactor assembly
according to the present invention in addition to the gas/solid
separation means. Typically solid filter means are knock-out drums.
The solid filter means are not comprised in the term gas/solid
separation means. Accordingly, restriction of the present invention
to only one gas/solid separation unit in the gas/solid separation
means does not exclude the additional presence of the solid filter
means (41) in the reactor assembly.
The flow through device (29) allows for varying the amount of a
stream of particles, gas or fluid or mixtures thereof flowing
through the device. The variation occurs by adjusting the flow
through device. Thereby the flow through device lets pass 0 to 100%
of a stream in a certain direction. Furthermore, the flow through
device may additionally allow for passing the rest 100 to 0% of the
stream in at least one additional direction. Furthermore, the flow
through device (29) is preferably capable of sending and/or
receiving signals to/from the controller (31).
Usually the flow through device comprises a valve. Particularly,
the valve can be a one-way valve or a multiport valve. Various
valve designs are well known in the art.
A controller (31) is any kind of device allowing for receiving and
processing data and receiving and sending signals. Usually the
controller is a computer.
The fluidization gas stream of the present invention comprises
fluidization gas and may also comprise different amounts of solids.
Accordingly, the word "gas" does not necessarily exclude that
further components beside fluidization gas may be comprised in the
fluidization gas stream. However, the amount and nature of solids
comprised in the fluidization gas stream (34) and the gas
circulation line varies and depends, among others, on where the
content of solids is measured in the reactor assembly, as
additional process steps are effected on the stream, the operating
conditions of the fluidized bed reactor and the nature (e.g.
density) of the polymer produced in the reactor. For instance, in
the fluidization gas stream (34) in outlet line (33) more solids
are contained than in the fluidization gas stream in gas
circulation line (38), at the exit of the gas/solid separation
means.
Means for cooling (3) are required in view of the exothermic nature
of the polymerization reactions. Usually the means for cooling will
be in the form of a heat exchanger.
Means for pressurizing (4) enable the adjustment of the
fluidization gas velocity. They are typically compressors.
The fluidized bed reactor comprises a bottom zone (5), a middle
zone (6) and an upper zone (7). These zones form the actual
reaction zone denoting the room within the fluidized bed reactor
designated for the polymerization reaction. However, one skilled in
the art will understand that the polymerization reaction will go on
as long as the catalyst remains active and there is monomer to
polymerize. Thus chain growths can also occur outside the actual
reaction zone. For example, polymer collected in a collection
vessel will still polymerize further.
The terms bottom-, middle- and upper zone indicate the relative
position with respect to the base of the fluidized bed reactor. The
fluidized bed reactor vertically extends in upward direction from
the base, whereby the cross-section(s) of the fluidized bed reactor
are essentially parallel to the base.
The height of the fluidized bed reactor is the vertical distance
between two planes with the lower plane crossing the lowest point
of the bottom zone and the upper plane crossing the highest point
of the upper zone. The vertical distance denotes the distance along
a geometrical axis forming a 90.degree. angle with the base and
also the two planes, i.e. a gas entry zone (if present) shall as a
matter of definition contribute to the height of the fluidized bed
reactor. The height of the individual zones is the vertical
distance between the planes limiting the zones.
The term cross-section as used herein denotes the area of the
intersection with a plane being parallel to the base. If not
mentioned otherwise, the term cross-section always concerns the
inner cross-section without internals. For example if the middle
zone is cylindrical having an outer diameter of 4.04 m and the wall
of the cylinder has a thickness of 0.02 m, the inner diameter will
be 4.00 m, whereby the cross-section will be
2.0.times.2.0.times..pi. m.sup.2.apprxeq.12.6 m.sup.2.
The term free cross-section denotes the area of the total
cross-section allowing interchange of gases and particles. In other
words, in a sectional drawing with the section going through the
plane formed by the interphase plane of the cross-section of the
bottom zone and the cross-section of the middle zone, the free
cross-section is the area, which is unobstructed.
Having an essentially constant equivalent cross-sectional diameter
denotes an equivalent cross-sectional diameter having a variation
of below 5%.
Variation shall mean the difference of the equivalent
cross-sectional diameter maximum and the equivalent cross-sectional
diameter minimum versus the average equivalent diameter. For
example, if the maximum equivalent cross-sectional diameter was
4.00 m, the minimum equivalent cross-sectional diameter was 3.90 m,
and the average equivalent cross-sectional diameter was 3.95 m
variation would be (4.00-3.90) m/3.95 m=0.025, i.e. 2.5%.
Monotonically decreasing is to be understood in a mathematical
sense, i.e. the average diameter will decrease or will be constant
with respect to the flow direction of the fluidization gas through
the fluidized bed reactor. Monotonically decreasing equivalent
cross-sectional diameter includes two situations namely the
decrease of the equivalent cross-sectional diameter with respect to
the flow direction of the fluidization gas through the fluidized
bed reactor and also constancy of the equivalent cross-sectional
diameter with respect to the flow direction of the fluidization
gas. It should be understood, however, that even though a zone
having a monotonically decreasing diameter in the direction of flow
may have sections having an essentially constant diameter, the
diameter at the downstream end of the zone is always smaller than
the diameter at the upstream end of the zone.
By "strictly monotonically decreasing" it is meant that the
equivalent cross-sectional diameter will decrease with respect to
the flow direction of the fluidization gas through the fluidized
bed reactor. Thus, if a zone has a strictly monotonically
decreasing diameter in the direction of flow then at any point h of
the zone the diameter is smaller than at any other point upstream
of said point h.
The phrases "monotonically increasing" and "strictly monotonically
increasing" are to be understood correspondingly.
Equivalent cross-sectional diameter is the normal diameter in case
of circular cross-section. If the cross-section is not circular,
the equivalent cross-sectional diameter is the diameter of a circle
having the same area as the cross-section of the non-circular
cross-section embodiment.
As a matter of definition, the three reaction zones, bottom zone,
middle zone and upper zone shall differentiate as to their
equivalent cross-sectional diameter. In other words, the boundary
plane delimiting bottom zone and middle zone shall be the plane,
where the cross-sectional diameter changes from increasing values
to essentially constant values. The boundary plane delimiting
middle zone and upper zone shall be the plane, where the
cross-sectional diameter changes from essentially constant values
to decreasing values. In the subsequent text "diameter" is also
used in the meaning of "equivalent cross-sectional diameter" for
non-circular surfaces.
Cone geometry plays an important role for the present invention. A
cone is a three-dimensional geometric shape that tapers smoothly
from a flat to the apex. This flat usually will be a circle but may
also be elliptic. All cones also have an axis which is the straight
line passing through the apex, about which the lateral surface has
a rotational symmetry.
From a more functional perspective, the fluidized bed reactor
according to the present invention includes a gas entry section, a
first domain, wherein the superficial gas velocity of the
fluidization gas is essentially constant, and a second domain being
located above the first domain, wherein the superficial gas
velocity of the fluidization gas is higher relative to the first
domain, an inlet for the fluidization gas located in the gas entry
section, an outlet for the fluidization gas located in the second
domain; the outlet for the fluidization gas being coupled with the
fluidized bed reactor via a gas circulation line; and means for
separation of solids from gas being connected to said gas
circulation line.
Gas entry section denotes the part of the whole apparatus, where
the feed takes place and the bed is formed. The gas entry section
insofar differentiates from the so called first domain and second
domain.
The first domain denotes the part of the fluidized bed reactor,
where the superficial gas velocity of the fluidization gas is
essentially constant.
The second domain is located vertically above the first domain and
denotes the part of the fluidized bed reactor, where the
superficial gas velocity of the fluidization gas is higher than in
the superficial gas velocity in the first domain.
Gas velocity shall mean the superficial gas velocity.
DESCRIPTION
The new reactor assembly has various advantages. In a first aspect,
there is no disengagement zone. This leads to an economical
construction. The reactor can be operated so that the bed occupies
almost the total volume of the reactor. This enables higher
output/reactor size ratios further leading to substantial cost
reduction. Further the polymer is evenly distributed within the
reactor over the bed area and is accompanied by better coalescence
of gas bubbles. It further has been surprisingly found that the
solids flow vicinal to the walls of the reactor is high which leads
to a constant cleaning of the walls particularly in the upper zone.
In another aspect, it has been surprisingly found that within the
reactor assembly the entrainment of fines with the fluidization gas
is reduced as the undesirably large bubbles are destroyed. Further,
the heat removal from the polymer as a function of bed height is
more even and there is a better dispersion between the gas and the
polymer as in the prior art reactors and processes.
A further important advantage of the present invention is that the
separation of the polymer from the fluidization gas, for instance
by using one or more cyclone(s), can easily be done due to a high
concentration of solids in the fluidization gas. It has been
surprisingly found that the solids content in the fluidization gas
after the gas/solid separation is much lower in the present
invention compared with a plant/process resulting in a feed to the
gas/solids separation means characterized by a lower amount of
solids. In other words, the relatively high amount of solids before
the gas/solid separation in the present invention surprisingly
results to a better degree of separation of solids.
A further important advantage of the present invention is that the
separation of the polymer from the fluidization gas, for instance
by using one or more cyclone(s), can easily be done since the
weight fraction of fines contained in the solids is reduced. As
defined above fines are solids of a size below defined threshold.
The fines have a very small mass due to their small size. The
efficiency of the gas/solid separation means increases with the
mass of particles to be separated from the gas. For example, a
cyclone acts by taking advantage of centrifugal forces its
efficiency increases with the mass of particles to be separated
from the gas. It has been surprisingly found that the amount of
fines is low produced in the fluidized bed reactor according to the
present invention.
Moreover, a narrow particle size distribution (PSD) automatically
reduces the amount of fines as the threshold for fines practically
is significantly away from the typical maximum of a (narrow)
particle size distribution curve.
Furthermore, the mean particle size is simultaneously increased
associated with an increasing mean mass of the particles. It has
been surprisingly found that the recycling of solids, especially
fines back to the fluidized bed reactor the mean particle size is
enlarged.
Consequently a reduced amount of fines is entrained to the cyclone
resulting in an increased efficiency of the cyclone, i.e. an even
better degree of separation of solids from gas due to the present
invention. In other words, the fluidized bed reactor and the
gas/solid separation means, e.g. one or more cyclone(s), contribute
to the solution in a synergistic way.
It is preferred that the reactor assembly according to the present
invention comprises an inlet for the catalyst or catalyst
containing prepolymer. In the simplest embodiment, the catalyst or
catalyst containing prepolymer may be fed via the inlet for the
fluidization gas. However, a separate inlet for the catalyst or
catalyst containing prepolymer allows good mixing of the catalyst
into the bed. Most preferably the catalyst is fed to the most
turbulent zone.
In one embodiment, the reactor assembly according to the present
invention preferably comprises an outlet for the removal of sheets,
chunks and lumps. Though the formation rate for sheets, chunks and
lumps is extremely low, it is not possible to suppress the
formation thereof to zero under all reaction conditions. If present
the outlet for the removal of sheets, chunks and lumps will be
preferably located in the lowest part of the bottom zone. In the
most preferred embodiment, the outlet will be positioned in the
centre of the bottom zone. When the bottom zone has conical shape,
the outlet will preferably fall together with the apex of the
cone.
In a second embodiment, the outlet for the removal of sheets,
chunks and lumps is accompanied by means for the break-up of
sheets, chunks and/or lumps. Such means for break-up of sheets,
chunks and/or lumps are commercially available and they are
discussed, among others, in Stolhandske, Powder and Bulk
Engineering, July 1997 issue on pages 49-57 and Feldman, Powder and
Bulk Engineering, June 1987 issue on pages 26-29 both documents
being incorporated by reference herewith.
As explained above, the fluidized bed reactor according to the
present invention comprises three zones, a bottom zone (5), a
middle zone (6) and an upper zone (7).
In a first and preferred embodiment the fluidized bed reactor
according to the present invention consists of three zones, a
bottom zone (5), a middle zone (6) and an upper zone (7).
In a second embodiment, the fluidized bed reactor according of the
present invention comprises more than three zones, a bottom zone
(5), a middle zone (6) and an upper zone (7) and at least one
additional zone, whereby this at least one additional zone is
located below the bottom zone (5) with respect to the flow
direction of the fluidization gas.
The following applies to all embodiments of the invention.
The equivalent cross-sectional diameter of the upper zone
preferably is strictly monotonically decreasing with respect to the
flow direction of the fluidization gas, i.e. generally in upwards
vertical direction.
The middle zone of the fluidized bed reactor typically will be in
the form of a circular cylinder being denoted herein simply
cylinder. However, it is possible that the middle zone of the
fluidized bed reactor is in the form of an elliptic cylinder. Then
the bottom zone preferably is preferably in the form an oblique
cone. Then more preferably the upper zone is also in the form of an
oblique cone.
From a more functional perspective, the middle zone will
essentially form the first domain wherein the superficial gas
velocity of the fluidization gas is essentially constant. The upper
zone will essentially form the second domain wherein the
superficial gas velocity of the fluidization gas is higher relative
to the first domain.
The upper zone of the reactor assembly according to the present
invention is preferably shaped such that a gas-particle stream
vicinal to the inner walls is created, whereby the gas-particle
stream is directed downwards to the base. This gas-particle stream
leads to an excellent particle-gas distribution and to an excellent
heat balance. Further the high velocity of the gas and particles
vicinal to the inner walls minimizes lump- and sheet formation.
It is further preferred that the ratio of the height of the upper
zone to the diameter of the middle zone is within the range of from
0.3 to 1.5, more preferably 0.5 to 1.2 and most preferably 0.7 to
1.1.
It is particularly preferred that the reactor assembly according to
the present invention includes an upper zone being cone-shaped and
a middle zone being cylindrical shaped. The cone forming the upper
zone preferably is a right circular cone and the cylinder forming
the middle zone preferably is a circular cylinder.
More preferably the cone-angle of the cone-shaped upper zone is
10.degree. to 50.degree., most preferably 20 to 40.degree.. As
defined above, the cone-angle is the angle between the axis of the
cone and the lateral area (FIG. 3).
The specific cone-angles of the cone-shaped upper zone further
improve the tendency for back-flow of the particles countercurrent
to the fluidization gas. The resulting unique pressure balance
leads to an intensive break up of bubbles, whereby the
space-time-yield is further improved. Further as mentioned above,
the wall flow velocity, i.e., the velocity of particles and gas
vicinal to the inner walls is high enough to avoid the formation of
lumps and sheets.
The reactor assembly according to the present invention preferably
has a bottom zone shaped such that the particles distribute the gas
over the whole cross-section of the bed. In other words, the
particles act as a gas distribution grid. In the bottom zone gas
and solids are mixed in highly turbulent conditions. Because of the
shape of the zone, the gas velocity gradually decreases within said
bottom zone and the conditions change so that a fluidized bed is
formed.
The following specifically preferred reactor geometries can be
combined with the aforementioned first embodiment consisting of
three zones a bottom zone (5), a middle zone (6) and an upper zone
(7) and the second embodiment including at least one additional
zone, whereby this zone or these zones is/are located below the
bottom zone.
Preferably, the equivalent cross-sectional diameter of the bottom
zone (5) is monotonically increasing with respect to the flow
direction of the fluidization gas through the fluidized bed
reactor. As the flow direction of the fluidization gas is upwards
with respect to the base, the equivalent cross-sectional diameter
of the bottom zone is vertically monotonically increasing.
Monotonically increasing is to be understood in a mathematical
sense, i.e. the average diameter will increase or will be constant
with respect to the flow direction of the fluidization gas through
the fluidized bed reactor.
The equivalent cross-sectional diameter of the bottom zone
preferably is strictly monotonically increasing with respect to the
flow direction of the fluidization gas through the reactor, i.e.
generally vertically upwards.
More preferably, the bottom zone is cone-shaped and the middle zone
is cylindrical shaped.
The bottom zone preferentially has right circular cone shape and
the middle zone is in the form of a circular cylinder.
Alternatively the middle zone is in the form of an elliptic
cylinder and the bottom and the upper zone are in the form oblique
cones.
More preferably, the cone-angle of the cone-shaped bottom zone is
5.degree. to 30.degree., even more preferably 7.degree. to
25.degree. and most preferably 9.degree. to 18.degree., whereby the
cone-angle is the angle between the axis of the cone and the
lateral surface (FIG. 2).
It is further preferred that the equivalent diameter of the bottom
zone increases from about 0.1 to about 1 meters per one meter of
height of the bottom zone (m/m). More preferably, the diameter
increase from 0.15 to 0.8 m/m and in particular from 0.2 to 0.6
m/m.
The preferred cone-angles lead to additional improved fluidization
behaviour and avoid the formation of stagnant zones. As a result,
the polymer quality and stability of the process are improved.
Especially, a too wide cone-angle leads to an uneven fluidization
and poor distribution of the gas within the bed. While an extremely
narrow angle has no detrimental effect on the fluidization
behaviour it anyway leads to a higher bottom zone than necessary
and is thus not economically feasible.
However, as mentioned above, in a second embodiment, there is an at
least one additional zone being located below the bottom zone. It
is preferred that the at least one additional zone, or if there is
more than one additional zone, the total of the additional zones
contributes/contribute to a maximum of 15% to the total height of
the reactor, more preferably 10% to the total height of the reactor
and most preferably less than 5% of the total height of the
reactor. A typical example for an additional zone is a gas entry
zone.
Preferably, there is an unobstructed passageway in the direction of
flow of the fluidization gas through the fluidized bed reactor
between the bottom zone (5) and the upper zone (7). An unobstructed
passageway includes all geometries which allow substantially free
exchange of gas and particles between and within said zones. An
unobstructed passageway is characterized by the absence of
internals such as distribution plates and/or grids resulting in
substantially increased flow resistivity. Accordingly, the
fluidized bed reactor (1) of the present invention preferably
comprises no gas distribution grid and/or plates. An unobstructed
passageway is characterized by a ratio of the free
cross-section/total cross-section with respect to the partition
between the bottom zone and the middle zone of at least 0.95,
whereby the free cross-section is the area allowing interchange of
gases and whereby the total cross-section is the area of the inner
reactor cross section limited by the walls of the fluidized bed
reactor.
This shall be explained by way of an example. When the middle zone
has cylindrical form with an inner diameter of 4 meter, the total
cross-section is about 2.0.times.2.0.times..pi.
m.sup.2.apprxeq.12.6 m.sup.2. If the area of the free
cross-section, i.e. the area allowing interchange of gases is at
least 12.0 m.sup.2 the criteria for an unobstructed passageway will
be fulfilled. A typical example for an internal leading to a small
reduction as to the cross-section allowing interchange of gases and
solids is a vertical pipe. Such a pipe or a plurality of pipes
directs flow and insofar has guiding function. However, as the wall
thickness of the pipe (and fasteners) only limit the cross-section
to a very small degree, the interchange of gases and solids will be
essentially not limited.
The fluidized bed reactor assembly according to the present
invention can be used for producing polymers in a commercial scale,
for instance with a production capacity of from 2 to 40 tons per
hour or 10 to 30 tons per hour.
The reactor assembly according to the present invention preferably
includes means for injection of the fluidization gas with an
injection angle within the range of 120.degree. to 150.degree. with
respect to the vertical axis of the fluidized bed reactor. The
vertical axis forms a 90.degree. angle with the base. More
preferably the means for injection of the fluidization gas enable
an injection angle in the range of 130.degree. to 140.degree..
Moreover the reactor assembly according to the present invention
preferably comprises an outlet for the polymer. In the simplest
variant of the reactor assembly, the polymer can be withdrawn via
the fluidization gas stream and the gas/solid separation means,
e.g. the one or more cyclone(s).
The outlet for the polymer preferably is located in the middle
zone. More preferably the outlet is in the form of a nozzle.
Typically there will be numerous nozzles located in the middle
zone.
Advantageously, at least a part of the polymer is withdrawn
directly from the fluidized bed, meaning that the outlet nozzle
withdraws polymer from a level which is above the base of the
fluidized bed but below the upper level of the fluidized bed.
Hence, the outlet nozzles for withdrawing polymer are located in
the middle zone of the reactor. It is preferred to withdraw the
polymer continuously, as described in WO 00/29452. It is then also
possible to withdraw a part of the polymer via the fluidization gas
stream and the gas/solid separation means, e.g. the one or more
cyclone(s). The polymer obtained directly from the fluidized bed
and the polymer obtained via the fluidization gas stream and the
gas/solid separation means are then usually combined.
The circulation gas is cooled in order to remove the heat of
polymerization. Typically, this is done in a heat exchanger. The
gas is cooled to a temperature which is lower than that of the bed
to prevent the bed from heating because of the reaction. It is
possible to cool the gas to a temperature where a part of it
condenses. When the liquid droplets enter the reaction zone they
are vaporised. The vaporisation heat then contributes to the
removal of the reaction heat. This kind of operation is called
condensed mode and variations of it are disclosed, among others, in
WO-A-2007/025640, U.S. Pat. No. 4,543,399, EP-A-699213 and
WO-A-94/25495. It is also possible to add condensing agents into
the recycle gas stream, as disclosed in EP-A-696293. The condensing
agents are non-polymerizable components, such as n-pentane,
isopentane, n-butane or isobutane, which are at least partially
condensed in the cooler.
When producing olefin polymers in the presence of olefin
polymerization catalysts the superficial gas velocity in the middle
zone of the reactor is suitably within a range of from 5 to 80 cm/s
(or, from 0.05 to 0.8 m/s), preferably from 0.07 to 0.7 m/s, such
as 0.1 to 0.5 m/s or 0.3 m/s or 0.2 m/s or 0.1 m/s.
The reactor may be used for polymerizing monomers in the presence
of a polymerization catalyst. Monomers which can thus be
polymerized include olefins, diolefins and other polyenes. The
reactor may thus be used to polymerize ethylene, propylene,
1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene,
1-octene, 1-decene, styrene, norbornene, vinyl norbornene,
vinylcyclohexane, butadiene, 1,4-hexadiene, 4-methyl-1,7-octadiene,
1,9-decadiene and their mixtures. Especially, the reactor is useful
in polymerizing ethylene and propylene and their mixtures,
optionally together with other alpha-olefin comonomers having from
4 to 12 carbon atoms.
In addition to the monomers different co-reactants, adjuvants,
activators, catalysts and inert components may be introduced into
the reactor.
Any polymerization catalyst may be used to initiate and maintain
the polymerization. Such catalysts are well known in the art.
Especially the catalyst should be in the form of a particulate
solid on which the polymerization takes place. Suitable catalysts
for olefin polymerization are, for instance, Ziegler-Natta
catalysts, chromium catalysts, metallocene catalysts and late
transition metal catalysts. Also different combinations of two or
more such catalysts, often referred to as dual site catalysts, may
be used.
Examples of suitable Ziegler-Natta catalysts and components used in
such catalysts are given, for instance, in WO-A-87/07620,
WO-A-92/21705, WO-A-93/11165, WO-A-93/11166, WO-A-93/19100,
WO-A-97/36939, WO-A-98/12234, WO-A-99/33842, WO-A-03/000756,
WO-A-03/000757, WO-A-03/000754, WO-A-03/000755, WO-A-2004/029112,
WO-A-92/19659, WO-A-92/19653, WO-A-92/19658, U.S. Pat. No.
4,382,019, U.S. Pat. No. 4,435,550, U.S. Pat. No. 4,465,782, U.S.
Pat. No. 4,473,660, U.S. Pat. No. 4,560,671, U.S. Pat. No.
5,539,067, U.S. Pat. No. 5,618,771, EP-A-45975, EP-A-45976,
EP-A-45977, WO-A-95/32994, U.S. Pat. No. 4,107,414, U.S. Pat. No.
4,186,107, U.S. Pat. No. 4,226,963, U.S. Pat. No. 4,347,160, U.S.
Pat. No. 4,472,524, U.S. Pat. No. 4,522,930, U.S. Pat. No.
4,530,912, U.S. Pat. No. 4,532,313, U.S. Pat. No. 4,657,882, U.S.
Pat. No. 4,581,342, U.S. Pat. No. 4,657,882, EP-A-688794,
WO-A-99/51646, WO-A-01/55230, WO-A-2005/118655, EP-A-810235 and
WO-A-2003/106510.
Examples of suitable metallocene catalysts are shown in
WO-A-95/12622, WO-A-96/32423, WO-A-97/28170, WO-A-98/32776,
WO-A-99/61489, WO-A-03/010208, WO-A-03/051934, WO-A-03/051514,
WO-A-2004/085499, EP-A-1752462, EP-A-1739103, EP-A-629631,
EP-A-629632, WO-A-00/26266, WO-A-02/002576, WO-A-02/002575,
WO-A-99/12943, WO-A-98/40331, EP-A-776913, EP-A-1074557 and
WO-A-99/42497,
The catalysts are typically used with different activators. Such
activators are generally organic aluminium or boron compounds,
typically aluminium trialkyls, alkylaluminium halides, alumoxanes
In addition different modifiers, such as ethers, alkoxysilanes, and
esters and like may be used.
Further, different coreactants, may be used. They include chain
transfer agents, such as hydrogen and polymerization inhibitors,
such as carbon monoxide or water. In addition, an inert component
is suitably used. Such inert component may be, for instance,
nitrogen or an alkane having from 1 to 10 carbon atoms, such as
methane, ethane, propane, n-butane, isobutane, n-pentane,
isopentane, n-hexane or like. Also mixtures of different inert
gases may be used.
The polymerization is conducted at a temperature and pressure where
the fluidization gas essentially remains in vapour or gas phase.
For olefin polymerization the temperature is suitably within the
range of from 30 to 110.degree. C., preferably from 50 to
100.degree. C. The pressure is suitably within the range of from 1
to 50 bar, preferably from 5 to 35 bar.
The reactor is preferably operated in such conditions that the bed
occupies at least 70% of the combined volume of the middle zone and
the upper zone, more preferably at least 75% and most preferably at
least 80%. The same numbers hold for the inventive processes
according to the present invention. When the reactor is operated in
this manner it has been found that surprisingly the bubbles break
up at the upper part of the reactor or are prevented from growing.
This is advantageous for a number of reasons. First, when the
volume occupied by the bubbles is reduced, the volume of the
reactor is more effectively used for the polymerization and the
"dead" volume is reduced. Second, the absence of large bubbles
reduces the entrainment of fines from the reactor. Instead, the
polymer that is carried out of the reactor with the fluidization
gas represents the total polymer within the reactor. Therefore, it
is possible to separate the polymer from the fluidization gas, for
instance by using a cyclone, and withdraw this polymer as the
product or direct it into further polymerization stages. In
addition to that, the separated solids can be recycled back to the
fludized be reactor. Third, even though polymer particles are
entrained from the reactor together with the fluidization gas, the
polymer is surprisingly easier to separate from the fluidization
gas than if the amount of polymer were smaller. Fourth, the polymer
particles obtained from the reactor with the fluidization gas
surprisingly contain a minimized fraction of fines. Therefore, when
the fluidization gas withdrawn from the top of the reactor is
passed through a cyclone the resulting overhead stream surprisingly
contains a smaller amount of polymer particles than in a
conventional fluidized bed reactor equipped with a similar cyclone.
Thus the reactor assemblies and the processes according to the
present invention combine a fluidized bed reactor and means for
separation of solids/gas in a synergistic way. Furthermore, the
underflow stream has better flow properties and is less prone for
plugging than in a similar conventional process.
The fluidization gas is withdrawn from the upper zone of the
fluidized bed reactor. For withdrawal of the fluidization gas one
or more outlets are provided in the upper zone, more preferably,
these one or more outlets are located at a height of more than 90%
of the total height of the fluidized bed reactor.
Furthermore, the one or more outlets are preferably located in the
upper zone all at the same height or all at different heights or a
combination of both on the fluidized bed reactor.
Preferably, at least one of the one or more outlets is/are located
at a height of more than 95% of the total height of the fluidized
bed reactor.
More preferably one outlet is located at the highest port of the
reactor and the other outlets, if present, are located at a height
of more than 90% of the total height of the fluidized bed reactor,
even more preferably one outlet is located at the highest port of
the reactor and the other outlets, if present, are located at a
height of more than 95% of the total height of the fluidized bed
reactor and most preferably, only one outlet for withdrawal of
fluidization gas is present in the upper zone of the fluidized bed
reactor and is located at the highest port of the reactor.
The one or more outlets are connected to one or more outlet lines.
The outlet lines convey the fluidization gas stream to the
gas/solid separation means comprising one or more gas/solid
separation units. The number of outlets can, but need not equal the
number of gas/solid separation units. For instance, when some or
all of the more outlet lines are merged fluidization gas from a
certain number of outlets can be conveyed to a minor number of
gas/solid separation unit. Preferably, only one gas/solid
separation unit is present in the gas/solid separation means.
Furthermore, the one or more gas/solid separation units are
preferably one or more cyclones. More preferably only one cyclone
is present in the gas/solid separation means.
As already outlined above, the one or more outlets in the upper
zone of the reactor are connected to one or more outlet lines. The
outlet lines convey the fluidization gas stream (34) to the
gas/solid separation means comprising one or more gas/solid
separation units. Hence, in case one or more outlets in the upper
zone of the reactor are present and connected to one or more outlet
lines the entirety of streams conveyed through the one or more
outlet lines is the fluidization gas stream (34).
From the gas/solids separation means an overhead stream and solid
recycling stream is taken. The overhead stream contains less solids
by weight than the solid recycling stream.
Preferably, the overhead stream contains less than 5.0% by weight,
more preferably less than 3.0% and even more preferably less than
1.0% by weight, even more preferably less than 0.75% and most
preferably less than 0.5% by weight of solids. Preferably, the gas
amount in the overhead stream is more than 95.0%, more preferably
more than 97.0%, even more preferably more than 99.0% even more
preferably more than 99.25% and most preferably more than 99.5% by
weight.
The solid recycling stream (36), typically contains mainly solid
material and includes some gas between the particles. Accordingly
the solid recycling stream contains the majority of the mass of the
polymer particles that were entrained from the fluidized bed
reactor with the fluidization gas stream (34) Typically the solid
recycling stream (36) contains at least 75%, preferably 80% and
more preferably 85% by weight solids and only at most 25%,
preferably 20% and most preferably 15% by weight gas.
As discussed above, the gas/solid separation means may comprise one
or more gas/solid separation units.
In case the gas/solid separation means contain only one gas/solid
separation unit the unit overhead stream is identical to the
overhead stream and the unit solid recycling stream is identical to
the solid recycling stream.
From each gas/solid separation unit a unit overhead stream and a
unit solid recycling stream is obtained.
In case of the gas/solid separation unit being a cyclone the unit
overhead stream is taken from the top outlet of the cyclone and the
unit solid recycling stream, is the underflow of the cyclone taken
from the bottom outlet of the cyclone.
In case two or more gas/solid separation units are present, these
gas/solid separation units may be either arranged in parallel or
series.
In the following preferred variants of a parallel assembly of
gas/solid separation unit are provided.
In a parallel assembly, usually and preferably, the unit overhead
stream of each gas/solid separation unit contains less than 5.0% by
weight, more preferably less than 3.0% and even more preferably
less than 1.0% by weight, even more preferably less than 0.75% and
most preferably less than 0.5% by weight of solids. Preferably, the
gas amount in the unit overhead stream of each gas/solid separation
unit is more than 95.0%, more preferably more than 97.0%, even more
preferably more than 99.0% even more preferably more than 99.25%
and most preferably more than 99.5% by weight.
In a parallel assembly, usually and preferably, the unit solid
recycling stream contains mainly solid material and includes some
gas between the particles.
Accordingly the unit solid recycling stream contains the majority
of the mass of the polymer particles that were entrained from the
fluidized bed reactor with the fluidization gas stream (34)
Typically the unit solid recycling stream contains at least 75%,
preferably 80% and more preferably 85% by weight solids and only at
most 25%, preferably 20% and most preferably 15% by weight gas.
As already outlined above, the one or more outlets in the upper
zone of the reactor are connected to one or more outlet lines. The
outlet lines convey the fluidization gas stream (34) to the
gas/solid separation means comprising one or more gas/solid
separation units.
In a first and preferred variant of the case wherein the gas/solid
separation units are arranged in parallel, and only one outlet line
which conveys the fluidization gas stream (34) to the gas/solid
separation means as mentioned in the foregoing paragraph is
present, or the streams of all outlet lines as mentioned in the
foregoing paragraph are combined prior to being fed to the
gas/solid separation means; the fluidization gas stream (34) is
split and a portion thereof is fed to each gas/solid separation
unit. In this regard portion denotes a part of the whole which does
not differ in its physical properties from another portion. As
mentioned above, preferably only one outlet line for conveying the
fluidization gas stream (34) is present. Hence, in the first and
preferred variant of the case wherein the gas/solid separation
units are arranged in parallel preferably only one outlet line
which conveys the fluidization gas stream (34) to the gas/solid
separation means as mentioned in the foregoing paragraph is present
and the fluidization gas stream (34) is split and a portion thereof
is fed to each gas/solid separation unit.
In this variant the splitting or combination and subsequent
splitting of the streams in the outlet line(s) such that the
gas/solid separation is effected in more than one gas/solid
separation unit allows the usage of smaller gas/solid separation
units. Furthermore, in case one gas/solid separation unit fails it
is not necessary to completely stop the process but replacement of
gas/solid separation unit during operation is possible.
Furthermore, in case two or more outlets in the upper zone of the
reactor are present at different heights the respective streams
withdrawn usually have a different composition allowing for
fine-tuning of the fluidisation gas stream (34). Thus, optionally,
valves are present in each outlet line.
In a second variant of the case wherein the gas/solid separation
units are arranged in parallel and two or more outlet lines which
convey the fluidization gas stream (34) to the gas/solid separation
means are present each of said outlet lines maybe connected to a
different gas/solid separation unit. Although not being preferable,
it is also possible that the stream in each outlet line is further
split into two or more streams and each individual stream is fed to
a different gas/solid separation unit or in case more than two
outlet lines are present instead of being further split, the
streams of two outlet lines may also be combined and fed to a
gas/solid separation unit.
In case of a parallel arrangement of the gas/solid separation units
according to each of the above variants usually the unit overhead
streams of all gas/solid separation units are combined prior to
further treatment, e.g. in the solid filter means (41). Hence, the
combination is the overhead stream (42) obtained from the gas/solid
separation means.
Similarly, the unit solid recycling streams of all gas/solid
separation units in case of a parallel arrangement of the gas/solid
separation units are preferably combined prior to any further
treatment such as introduction into the flow through device, as
further described below. Although less preferred the unit solid
recycling streams of the gas/solid separation units in case of a
parallel arrangement of the gas/solid separation units may also be
introduced into individual flow through devices and the streams
obtained from each flow through device are combined.
More preferably, in case of a parallel arrangement of the gas/solid
separation units according to each of the above variants the unit
overhead streams of all gas/solid separation units are combined
prior to further treatment, e.g. in the solid filter means (41),
thereby forming the overhead stream obtained from the gas/solid
separation means; and the unit solid recycling streams of all the
gas/solid separation units are combined prior to introduction into
the flow through device, thereby forming the solid recycling stream
obtained from the gas/solid separation means.
Even more preferably, in case of a parallel arrangement of the
gas/solid separation units according to each of the above variants
only one outlet for withdrawal of fluidization gas is present in
the upper zone of the fluidized bed reactor and is located at the
highest port of the reactor and the unit overhead streams of all
gas/solid separation units are combined prior to further treatment,
e.g. in the solid filter means (41), thereby forming the overhead
stream obtained from the gas/solid separation means; and the unit
solid recycling streams of all the gas/solid separation units are
combined prior to introduction into the flow through device,
thereby forming the solid recycling stream obtained from the
gas/solid separation means.
Alternatively, the gas/solid separation units are connected in
series.
In the following preferred variants in case the gas/solid
separation units are arranged in series are described.
In a serial assembly, usually and preferably, the unit overhead
stream of each gas/solid separation unit contains less than 5.0% by
weight, more preferably less than 3.0% and even more preferably
less than 1.0% by weight, even more preferably less than 0.75% and
most preferably less than 0.5% by weight of solids. Preferably, the
gas amount in the unit overhead stream of each gas/solid separation
unit is more than 95.0%, more preferably more than 97.0%, even more
preferably more than 99.0% even more preferably more than 99.25%
and most preferably more than 99.5% by weight.
In a serial assembly, usually and preferably, the unit solid
recycling stream contains mainly solid material and includes some
gas between the particles. Accordingly the unit solid recycling
stream contains the majority of the mass of the polymer particles
that were entrained from the fluidized bed reactor with the
fluidization gas stream (34) Typically the unit solid recycling
stream contains at least 75%, preferably 80% and more preferably
85% by weight solids and only at most 25%, preferably 20% and most
preferably 15% by weight gas.
As already outlined above, the one or more outlets in the upper
zone of the reactor are connected to one or more outlet lines. The
outlet lines convey the fluidization gas stream (34) to the
gas/solid separation means comprising one or more gas/solid
separation units.
In case more than one line is present, the streams are combined to
form the fluidization gas stream (34).
The fluidization gas stream (34) is fed to a first gas/solid
separation unit.
This first unit overhead stream obtained from the first gas/solid
separation unit is fed to a second gas/solid separation unit. The
unit overhead stream of the second gas/solid separation unit is the
overhead stream of the gas/solid separation means.
The unit solid recycling streams obtained from the first and the
second gas/solid separation units are preferably combined prior to
any further treatment such as introduction into the flow through
device, thereby forming the solid recycling stream obtained from
the gas/solid separation means. Although less preferable, it is
also possible to introduce the first unit solid recycling stream
and the second unit recycling stream into separate flow through
devices and combine the streams obtained therefrom.
It is also possible to arrange three gas/solid separation units in
series whereby the inlet of the second and third gas/solid
separation unit is connected to the outlet of the unit overhead
stream of the respective upstream gas/solid separation unit. In
this variant the unit overhead stream of the third gas/solid
separation unit is the overhead stream of the gas/solid separation
means. In this variant the unit solid recycling streams obtained
from the first, second and third gas/solid separation units are
preferably combined prior to introduction into the flow through
device, thereby forming the solid recycling stream obtained from
the gas/solid separation means.
In such a serial arrangement the unit overhead stream obtained from
the first gas/solid separation unit is subjected to further
gas/solid separation steps.
Such an arrangement may be used in case a high amount of fines is
generated due to the nature of the polymer produced.
Of course combinations of serial and parallel arrangements are also
possible. Preferably, the gas/solid separation means contain only
one gas/solid separation unit; or two or more gas/solid separation
units whereby all gas/solid separation units are arranged in
parallel;
more preferably, the gas/solid separation means contain only one
gas/solid separation unit; or two or more gas/solid separation
units whereby all gas/solid separation units are arranged in
parallel and wherein the unit solid recycling streams of all
gas/solid separation units are combined to form the solid recycling
stream of the gas/solid separation means prior to any further
treatment such as introduction into the flow through device; and
the unit overhead streams of all gas/solid separation units are
combined to form the overhead stream of the gas/solid separation
means prior to any further treatment such as introduction into the
optional solid filter means (41)
and most preferably, the gas/solid separation means contain only
one gas/solid separation unit.
As already outlined above, in case the gas/solid separation means
contain only one gas/solid separation unit the unit overhead stream
is identical to the overhead stream and the unit solid recycling
stream is identical to the solid recycling stream.
As discussed above, the gas/solid separation is conveniently
performed by cyclones. Hence, the gas/solid separation units are
preferably cyclones.
When a cyclone is used, the unit overhead stream is taken from the
top outlet of the cyclone and the unit solid recycling stream, i.e.
the underflow of the cyclone, is taken from the bottom outlet of
the cyclone.
In a cyclone a gas stream containing solids enters a cylindrical or
conical chamber tangentially at one or more points. The gas leaves
as unit overhead stream or, in case only one cyclone is present in
the gas/solid separation means leaves as overhead stream (42)
through a central opening at the top, top outlet, of the cyclone
chamber and the solids as unit solid recycling stream (underflow)
through an opening at the bottom, bottom outlet, of the cyclone
chamber. The solids are forced by inertia towards the walls of the
cyclone from where they fall downwards.
In the present invention the stream of solids from the bottom
outlet of the cyclone is the unit solid recycling stream or, in
case only one cyclone is present in the gas/solid separation means
the solid recycling stream (36).
In the present invention the gas circulation line (38) runs from
the top outlet of the gas/solid separation means (2) to the inlet
for fluidization gas (8), Thus, in the gas circulation line (38)
further devices such as solid filter means (41), means for cooling
(3), means for pressurizing (4) etc. may be present.
The solid recycling stream is recycled via a solid recycling line,
gas/solid separation means to the solid recycling inlet back to the
fluidized bed reactor.
The solid recycling line includes a flow through device.
As already outlined above, the flow through device (29) allows for
varying the amount of a stream of particles, gas or fluid or
mixtures thereof flowing through the device. The variation occurs
by adjusting the flow through device. Thereby the flow through
device lets pass 0 to 100% of a stream in a certain direction.
Furthermore, the flow through device may additionally allow for
passing the rest 100 to 0% of the stream in at least one additional
direction.
The flow through device (29) in the solid recycling line supplies 0
to 100%, preferably 10 to 90%, more preferably 10 to 60% of the
solid recycling stream (36) upstream of the flow through device to
the fluidized bed reactor based on the volume of the solid
recycling stream (36) upstream of the flow through device.
Furthermore, the flow through device (29) may additionally allow
for branching off from the solid recycling stream (36) entering the
flow through device a stream to downstream process stages (40)
directed to down stream process stages through the line to
downstream process stages (39). It is possible to branch off up to
100% of the volume of the solid recycling stream (36) upstream of
the flow through device to the stream to downstream process stages
(40).
Accordingly the stream to downstream process stages (40) is 100 to
0%, preferably 90 to 10%, more preferably 90 to 40% based on the
volume of the solid recycling stream (36) upstream of the flow
through device.
Accordingly, the remaining solid recycling stream (36) downstream
of the flow through device (29) corresponds to the difference of
the solid recycling stream (36) upstream of the flow through device
minus the stream to downstream process stages (40).
The relation between the solid recycling stream downstream of the
flow through device (29) and the at least one further stream is
described by ratio of the flow rate of the solid recycling stream
downstream of the flow through device (29) to the flow rate of the
at least one further stream based on the volume of the streams.
The control of the volume of the solid recycling stream downstream
of the flow through device (29) with respect to the solid recycling
stream upstream of the flow through device (29) may be effected by
routing 100% of the solid recycling stream upstream of the flow
through device (29) to the fluidized bed reactor for a fraction of
the total time of the polymerization process and the remaining time
to other stream(s), e.g. the stream to downstream process stages
(40). In such a case the fractions of time are usually chosen to
obtain the above relations between the flow rate of the solid
recycling stream downstream of the flow through device to the flow
rate of the at least one further stream based on the volume of the
streams.
This is explained by the following non-limiting example.
Over a period of ten minutes of continuous polymerization where 90%
of the volume of the solid recycling stream upstream of the flow
through device (29) should be routed to the fluidized bed reactor,
then for nine minutes 100% of the volume of the solid recycling
stream upstream of the flow through device (29) is routed to the
fluidized bed reactor and for the remaining one minute 100% of the
volume of the solid recycling stream upstream of the flow through
device (29) is routed to other stream(s), such as the stream to
downstream process stages (40).
However, preferably, the flow through device allows for
simultaneously supplying a part of the volume of the solid
recycling stream upstream of the flow through device (29) to the
fluidized bed reactor and to other stream(s), such as the stream to
downstream process stages (40). Nevertheless during the process the
amount fed to each stream (=split) may be adjusted by the
controller.
In such a case the fractions of time and split are usually chosen
to obtain the above relations between the flow rate of the solid
recycling stream downstream of the flow through device to the flow
rate of the at least one further stream based on the volume of the
streams.
The flow through device preferably comprises a valve.
One alternative of a flow through device is a multiport valve. Such
a multiport valve usually contains at least three connections and
at least two settings.
These two settings can be on/off, i.e. either 100% of the volume of
the solid recycling stream (36) upstream of the flow through device
is routed to the fluidized bed reactor or 0% of the volume of the
solid recycling stream (36) upstream of the flow through device is
routed to the fluidized bed reactor.
In case 0% of the volume of the solid recycling stream (36)
upstream of the flow through device is routed to the fluidized bed
reactor the remaining 100% are routed to the line to downstream
process stages (39) to downstream process stages (40).
An example of a valve having on/off settings is a three-way ball
valve of L-type.
These two settings can also be 90/10 and 10/90, i.e. either 90% of
the volume of the solid recycling stream (36) upstream of the flow
through device is routed to the fluidized bed reactor and 10% of
the volume of the solid recycling stream (36) upstream of the flow
through device is routed to the stream to downstream process stages
(40) or 10% of the volume of the solid recycling stream (36)
upstream of the flow through device is routed to the fluidized bed
reactor and 90% of the volume of the solid recycling stream (36)
upstream of the flow through device is routed to the line to
downstream process stages (39) to downstream process stages
(40).
Alternative setting may be 60/40 and 10/90 or 60/40 and 40/60.
The multiport valve may also have more than two settings, e.g.
90/10; 60/40; 40/60 and 10/90.
Alternatively the multiport valve may allow for supplying a
variable percentage of the volume of the solid recycling stream
(36) upstream of the flow through device to the fluidized bed
reactor. Hence, the regulation of the volume of the solid recycling
stream to the fluidized bed reactor is not stepwise but can be
continuous between 0 and 100% with respect to the volume of the
solid recycling stream upstream of the flow through device.
Another alternative of the flow through device used in the present
invention comprises a simple branch point having one inlet and at
least two outlets, preferably the simple branch point is having one
inlet and two outlets.
In a first variant of said alternative the line to downstream
process stages (39) is connected to one of the outlets of the
simple branch point and a line L.sub.A is connected to a different
outlet of the simple branch point, the line L.sub.A is connected to
the inlet of a control valve and the outlet of the control valve is
connected to the fluidized bed reactor. This stream downstream of
the control valve ist the solid recycling stream (36) downstream of
the flow through device.
By adjusting the flow through the control valve automatically, the
flow through the line to downstream process stages (39) is also
adjusted.
In a second variant of said alternative the solid recycling line
(36) downstream of the flow through device is connected to one of
the outlets of the simple branch point and a line L.sub.B is
connected to a different outlet of the simple branch point, the
line L.sub.B is connected to the inlet of a control valve and the
outlet of the control valve is connected to the line to downstream
process stages (39).
In a third variant of said alternative a line L.sub.A is connected
to an outlet of the simple branch point, the line L.sub.A is
connected to the inlet of a first control valve and the outlet of
the first control valve is connected to the fluidized bed reactor.
This stream downstream of the first control valve ist the solid
recycling stream (36) downstream of the flow through device; a line
L.sub.B is connected to a different outlet of the simple branch
point, the line L.sub.B is connected to the inlet of a second
control valve and the outlet of the second control valve is
connected to the line to downstream process stages (39).
In case two outlets are present in any of the above three variants,
the simple branch point may be replaced by a three-way ball valve
of T-type which allows one inlet to be connected with either one or
both of the outlets. Thereby the settings 0/100 and 100/0 can be
effected directly at the branch.
More preferably, the flow through device is either a one-way valve
or a multiport valve. The multiport valve contains at least three
connections and at least two settings. The at least two settings
are not absolute. The least two settings merge, thereby providing
intermediate positions allowing the outflow of at least two solid
recycling streams having the same or different amounts.
Preferably the multiport valve is a 3/2-way valve. The flow through
device allows for varying the amount of solid recycling stream
flowing trough the device. Thereby the flow through device lets
pass 0 to 100%, preferably 10 to 60%, of solid recycling stream to
the fluidized bed reactor. Furthermore, the flow through device may
additionally allow for routing the rest 100 to 0%, preferably 90 to
40% of solid recycling stream to down stream process stages.
It is possible to introduce a gas stream into the solid recycling
stream (36) and to the solid stream to downstream process stages
(40) to facilitate the transport of the powder. The gas stream may
consist of inert gases, such as nitrogen or saturated hydrocarbons,
such as ethane, propane, butanes, pentanes and the like. It may,
however, also comprise or consist of the fluidization gas or
comprise one or more components forming the fluidization gas
together with one or more inert gases.
Preferably the adjusting of the flow through device occurs via a
controller.
Downstream process stages are further process or reaction steps.
Preferably, downstream process stages comprise at least the steps
of mixing the polymer with additives and extruding the polymer
comprising the additives into pellets. They may also comprise a
further reactor, means for cooling, means for pressurizing and/or
one or more outlets for the polymer. Preferably such further
reactor is a moving bed reactor allowing for a dual reactor
assembly. Dual reactor assemblies in general are well-known in the
art.
The polymer stream withdrawn from the fluidized bed reactor (1) via
outlet of the polymer (14) and the solid recycling stream branched
off to the output of the polymer (40) downstream from the flow
through device can be combined for product recovery.
Furthermore a controller (31) and the flow through device (29)
preferably communicate with each other by sending and receiving one
or more signals in at least one direction between the flow through
device (29) and the controller (31). Preferably, the one or more
signal is only sent by the controller and received by the flow
through device. However, it is also possible that the one or more
signal is only sent by the flow through device and received by the
controller. Preferably, the one or more signal is digital,
electric, mechanic, electromagnetic and/or a combination thereof.
More preferably the signal is digital. Due to that communication
the flow through device (29) is adjusted by the controller (31). As
a consequence the flow though device (29) varies and/or routes the
solid recycling stream (36) as outlined above.
The controller (31) is a device that receives and/or sends signals
to the flow through device as outlined above. Furthermore the
controller is a device that can receive data, process the data and
send signals to the fluidized bed reactor. Furthermore, preferably
the controller is a device that communicates with the fluidized bed
reactor. Preferably the controller is a computer. The received data
are preferably in digital form. The received data originate in the
measurement of the mean particle size and/or the particle size
distribution of an fluidization gas stream (34) from the fluidized
bed reactor and/or originate in the analysis of the operation
conditions in the fluidized bed reactor.
The particle size d.sub.p may be and preferably is measured as
follows.
The particle size d.sub.p is measured using a Beckman Coulter LS
200 Laser Diffraction Particle Size Analyser.
The samples were prepared by mixing the polymer powder with
isopropyl alcohol to a paste, which is further mixed in an ultra
sound bath for 20-30 seconds.
The paste is added to the sample unit of the Coulter instrument
which contains isopropyl alcohol. The recommended powder
concentration is 8 to 12%. The size of the sample unit is 125
ml.
The analysis is performed according to the computer program of the
software LS32, version 3.10.2002 of the instrument. The run length
is 60 seconds. The calculation of the results is made by the
software. Thereby the particle size distribution is obtained.
From the particle size distribution the characteristics such as the
median particle size, different average particle sizes, the
variance, the standard deviation, and the span can be
calculated.
Mass flow rate of solids may be determined by any method known in
the art. These include gravimetric methods and the methods based on
Coriolis force.
Weight fraction of solids may be also be determined by using any
applicable method, such as by taking samples and separating and
weighing the components; or, by determining the density of the
stream (for instance, by using Coriolis or radioactive methods),
analyzing the gas composition and then calculating the solids
content from the measured density of the mixture, calculated
density of the gas (calculated from the composition) and the known
density of the polymer.
The flow rates can be determined by any method known in the art,
such as methods based on Coriolis force; methods based on thermal
conductivity; methods based on pressure difference; and others.
Suitable apparatuses for the measurement of the mass flow rate of
solids flows are LB442 distributed by Berthold Technologies
(radioactive) and Multicor distributed by Schenck AccuRate
(Coriolis force).
A suitable apparatus for the measurement of the flow rate is Micro
Motion (e.g. Elite Coriolis meter) distributed by Emerson Process
Management.
The measurement of the gas composition is also well-known in the
art and is usually accomplished by on-line gas chromatography. A
suitable apparatus therefor is Maxum of Siemens.
In case the flow rates and the densities of the flows are known
(which both can be measured by Coriolis flow meters as already
outlined above), as well as the gas composition, the solids content
of the flow can be calculated from the densities of the gas which,
in turn is calculated from the composition, density of the solid,
i.e. the density of the polymer which is known and the density of
the mixed stream which has been measured.
Analysis of the operation conditions in the fluidized bed reactor
preferably comprises analysis of the parameters concerning
fluidization conditions. More preferably the analysis comprises the
measurement of the contents of the components of the fluidization
gas, such as the contents of monomer, hydrogen, comonomers and
eventual inert components, the flow rate of the fluidization gas,
the temperature and the pressure of the fluidization gas at various
points of the gas circulation line, the temperature and pressure at
various levels of the reactor and the contents of solids in the
fluidization gas stream.
Preferably, due to communication with the fluidized bed reactor the
controller ensures that the operation conditions in the fluidized
bed reactor are maintained over the full production period.
Preferably, the controller and the fluidized bed reactor assembly
preferably communicate with each other by sending and receiving one
or more signals in at least one direction between the fluidized bed
reactor and the controller. Preferably, the one or more signal is
only sent by the controller and received by the fluidized bed
reactor assembly. However, it is also possible that the one or more
signal is only sent by the fluidized bed reactor assembly and
received by controller. Preferably, the one or more signal is
digital, electric, pneumatic, electromagnetic and/or a combination
thereof. More preferably the signal is digital.
Preferably, the actual particle cut diameter d.sub.50, is
determined and compared with a predetermined threshold value for
the particle cut diameter (thres). As it is known in the art the
actual particle cut diameter, d.sub.50, is the diameter of the
particle that has a 50% probability of being collected by the
cyclone. The actual particle cut diameter d.sub.50 can be
determined experimentally by collecting solid samples over a given
period of time both from the solid recycling stream (36) and the
overhead stream (42). The particle size distribution is then
determined from the solid samples. Furthermore, the solid
concentration of the overhead stream (42) sample is determined. The
flow rates of the fluidization gas stream (34) entering the cyclone
and of the overhead stream (42) leaving the cyclone are measured as
well as the flow rate of the solid recycling stream (36).
Then, the mass flow rate of the solids having a particle diameter
d.sub.p in the overhead stream (42) can be obtained from: {dot over
(m)}.sub.p,g=w.sub.p,gc.sub.sQ.sub.g,
And the mass flow rate of solids having a particle diameter d.sub.p
in the solids recycling stream (36) can be obtained from: {dot over
(m)}.sub.p,s=w.sub.p,s{dot over (m)}.sub.s
Wherein {dot over (m)}.sub.p,g is the mass flow rate of the solids
with a particle size d.sub.p in the overhead stream (42) {dot over
(m)}.sub.p,s is the mass flow rate of the solids with a particle
size d.sub.p in the solid recycling stream (36) {dot over
(m)}.sub.s is the mass flow rate of all solids contained in the
solid recycling stream (36) w.sub.p,g is the weight fraction of
solids with a particle size d.sub.p in the solid sample of the
overhead stream (42) w.sub.p,s is the weight fraction of solids
with a particle size d.sub.p in the solid sample of the solid
recycling stream (36) c.sub.s is the concentration of all solids
contained in the overhead stream (42) Q.sub.g is the volumetric
flow rate of the overhead stream (42)
The efficiency .eta.(p) for a solid of a particle size d.sub.p
being captured is then
.eta..function. ##EQU00001##
And d.sub.50 can then be found by fitting the value of d.sub.50
against d.sub.p and .eta.(p) in the following equation:
.eta..function.dd ##EQU00002##
The efficiency of the cyclone can then be evaluated by comparing
the experimentally obtained actual value of d.sub.50 to a
predetermined upper threshold (thres). If the experimental value
d.sub.50 is higher than the predetermined value (thres), then the
efficiency is not at a desired level. Then the return flow of the
solid recycling stream (36) into the fluidized bed reactor is
increased. In case, a stream to downstream process stages is
branched off from the solid recycling stream, the volume of the
stream to downstream process stages is thereby concomitantly
decreased. Accordingly, the ratio of the flow rate of the solid
recycling stream to the flow rate of the stream to downstream
process stages is increased based on the volume of the streams.
Alternatively, it is also possible to predetermine lower threshold
values to the efficiency .eta.(p) and compare them directly with
the experimentally determined actual values. If the efficiency is
lower than the predetermined value the return flow of the solid
recycling stream is increased as discussed above.
The particle size analysis can be done off-line by sieving, or,
preferably, by using a particle counter, such as those manufactured
and sold by Beckman Coulter and Malvern. Such instruments can be
used in-line, where a sample is directed automatically to the
analyzer and measured. The data is then sent to the process
controller or process computer. Instruments may also be used
off-line, so that a sample is taken manually and then analyzed. The
data is then manually entered into the process computer or
controller.
Usually, the efficiency of gas/solid separation in the gas/solid
separation means is higher in case the mass fraction of solids in
the fluidization gas stream entering the gas/solid separation means
is higher.
Thus, in a further aspect of the invention the efficiency of the
gas/solid separation means is estimated by determining the amount
of solids entering the gas/solid separation means. If the amount of
solids in the fluidization gas stream entering the gas/solid
separation means is too low then the flow of polymer in the solids
recycling stream is increased. This gradually increases the level
of the fluidized bed in the reactor. This in turn increases the
flow rate of polymer particles entrained by the fluidization gas,
thereby increasing the efficiency of the gas/solid separation
means. Furthermore, thereby the mass fraction of fines in the
solids contained in the fluidization gas stream is reduced.
In a further aspect of the invention the solid recycling stream
(36) is increased when the content of fines, usually deduced from
the measured particle size distribution of an fluidization gas
stream (34) from the fluidized bed reactor (1) is larger than a
predetermined set point for the content of fines.
The variation of the flow of the solid recycling stream back to the
fluidized bed reactor is achieved by adjusting the flow through
device as outlined above. The adjustment of the flow through device
may be effected by a controller.
According to one embodiment of the invention the controller
includes a model of the process and thus can predict some process
variables. For instance, the controller predicts the mean particle
size, span of the particle size distribution and/or the whole
particle size distribution of the particles contained in the
fluidization gas stream using the data obtained in analyzing the
operation conditions in the fluidized bed reactor. As outlined
above, these predicted values may then be compared to the measured
mean particle size, span of particle size distribution and/or the
particle size distribution. Further, also other process variables,
such as the content of solids in the fluidization gas, both before
and after the cyclone, may be predicted. The predicted values may
then be adjusted so that they better fit with the measured values.
Methods to do this are well known in control engineering and
include, for instance, Extended Kalman Filter and Instrumental
Variables.
As discussed above it is preferred that the solid recycling stream
is increased when the content of fines according to the measurement
of the particle size distribution is larger than the predetermined
maximum level. Alternatively, the solid recycling stream may be
increased if the solids content in the fluidization gas stream is
too low. As discussed above these control actions may be based on
measured or predicted values.
Typically the fluidization gas enters into the gas entry zone below
the bottom zone of the fluidized fed reactor. In said gas entry
zone the gas and eventual polymer or catalyst particles are mixed
in turbulent conditions. The velocity of the fluidization gas is
such that the eventual catalyst or polymer particles contained
therein are transferred into the bottom zone. However, polymer
agglomerates, such as lumps or sheets, fall downwards and may be
thus removed from the reactor. In a typical embodiment the gas
entry zone is a pipe typically having a diameter such that the gas
velocity is higher than about 1 m/s, such as from 2 to 70 m/s,
preferably from 3 to 60 m/s. It is also possible that the gas entry
zone has an increasing diameter in the flow direction so that the
gas velocity at the upper part of the gas entry zone is lower than
at the bottom part.
In the preferred embodiment discussed above the gas enters from the
gas entry zone to the bottom zone. A gas entry zone as a matter of
definition shall not be seen as part of the reactor and insofar
shall not contribute to the height of the reactor. Within the
bottom zone the fluidized bed is formed. The gas velocity is
gradually reduced so that at the top of the bottom zone the
superficial gas velocity is from about 0.02 m/s to about 0.9 m/s,
preferably from 0.05 to about 0.8 m/s and more preferably from
about 0.07 to about 0.7 m/s, such as 0.5 m/s or 0.3 m/s or 0.2 m/s
or 0.1 m/s,
Further, usually, in the above-mentioned preferred embodiment the
superficial velocity of the fluidization gas decreases in the
bottom zone preferably so that the value of a, which is the
reciprocal of the square root of the superficial velocity,
expressed in m/s,
##EQU00003## wherein v is the superficial velocity of the
fluidization gas, increases by a value within the range of from
0.66 to 4.4 per one meter length of the bottom zone. More
preferably the value of a as defined above increases by a value
within the range of from 0.94 to 3.6, even more preferably from 1.2
to 2.5 per one meter length of the bottom zone. Naturally, the
value of a increases in the direction of the flow of the
fluidization gas within the bottom zone, that is, in the upwards
direction.
Preferably the superficial velocity of the fluidization gas
monotonously decreases within the bottom zone, remains at a
constant level within the middle zone and monotonously increases
within the upper zone. Especially preferably, the superficial
velocity increases as described above.
From a process perspective, the middle zone of the fluidized bed
reactor is maintained under conditions such that the superficial
gas velocity is from 5 to 80 cm/s, preferably 10 to 70 cm/s.
The polymerization catalyst can be fed directly or can originate
from a previous prepolymerization stage, the later being preferred.
The polymerization catalyst is preferably introduced into the
middle zone via the respective inlet. The withdrawal of the
reaction product is preferably continuous such as disclosed in
WO-A-00/29452.
In a preferred embodiment according to the present invention the
reactor assembly according to the present invention further
comprising a loop reactor upstream of said fluidized bed
reactor.
In the following the methods according to the present invention are
further described. The preferred ranges, definitions and dimensions
as discussed above with respect to the reactor also apply for the
processes and methods and are incorporated by reference
herewith.
Furthermore, preferably, the methods according to the present
invention are carried out in the reactor assembly according to the
present invention including all preferred embodiments thereof.
In the following the use according to the present invention is
further described. The preferred ranges, definitions and dimensions
as discussed above with respect to the reactor also apply for the
use and are incorporated by reference herewith.
Furthermore, preferably, the uses according to the present
invention are carried out in the reactor assembly according to the
present invention including all preferred embodiments thereof.
Furthermore, the present invention relates to the use of a
controller (31), a flow through device (29) and a solid recycling
line (35) in a reactor assembly for the production of polymers for
minimizing the amount of fines produced by a fluidized bed reactor
(1), the reactor assembly including a fluidized bed reactor
comprising a bottom zone (5), a middle zone (6) and an upper zone
(7), one or more outlets (9) for fluidization gas streams (34)
located in the upper zone (7), gas/solid separation means (2), a
flow through device (29), a solid recycling line (35), a solid
recycling inlet (37), a gas circulation line (38), an inlet (8) for
fluidization gas located in the bottom zone (5) and a controller
(31).
In the following the processes according to the present invention
are further described. The preferred ranges, and dimensions as
discussed above with respect to the reactor also apply for the
processes and are incorporated by reference herewith.
Furthermore, preferably, the processes according to the present
invention are carried out in the reactor assembly according to the
present invention including all preferred embodiments thereof.
The present invention relates to a process for the production of
polymers in the presence of a polymerization catalyst in a reactor
assembly including a fluidized bed reactor as described above. The
processes according to the present invention preferably concern the
polymerization of polyolefins. More preferably the polyolefins are
momoners selected from the group of ethylene, propylene, and
C.sub.4 to C.sub.12 alpha olefins.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional drawing of the reactor assembly including a
fluidized bed reactor.
FIG. 2 is a sectional drawing of the cone shaped bottom zone. The
cone-angle being the angle between the axis of the cone and the
lateral surface is shown.
FIG. 3 is a sectional drawing of the cone shaped upper zone.
FIG. 4 is sectional drawing of the reactor assembly including a
fluidized bed reactor and two gas/solid separation units connected
in series in the gas/solid separation means.
REFERENCE LIST
1 fluidized bed reactor 2 gas/solid separation means 3 means for
cooling 4 means for pressurizing 5 bottom zone 6 middle zone 7
upper zone 8 inlet (for fluidization gas) 9 outlet for fluidization
gas stream (34) 10 line for the recycling of solids 11 inlet for
catalyst or prepolymer 12 outlet for sheets, chunks, and lumps 13
means for break-up of sheets 14 outlets for the polymer in the
middle zone 27 top outlet for overhead stream (42) 28 bottom outlet
for stream of solids 29 flow through device 30 data 31 controller
32 signal 33 outlet line 34 fluidization gas stream 35 solid
recycling line 36 solid recycling stream 37 solid recycling inlet
38 gas circulation line 39 line to downstream process stages and/or
output for polymer 40 stream to downstream process stages and/or
output for polymer 41 solid filter means 42 overhead stream 43
first gas/solid separation unit 44 second gas/solid separation unit
45 unit bottom outlets of the first and second gas/solid separation
unit 46 unit bottom streams of the first and second gas/solid
separation unit 47 unit overhead stream of the first gas/solid
separation unit 48 top outlet for the unit overhead stream of the
first gas/solid separation unit
DETAILED DESCRIPTION WITH RESPECT TO THE DRAWINGS
The invention shall now be explained with respect to the
drawings.
According to FIG. 1 the reactor assembly according to the present
invention comprises a fluidized bed reactor (1) having a bottom
zone (5), a middle zone (6) and an upper zone (7),
The bottom zone (5) and the middle zone (6) (and also the upper
zone (7)) form an unobstructed passageway as there is no
distribution plate.
Furthermore, the equivalent cross-sectional diameter of the bottom
zone (5) being monotonically increasing with respect to the flow
direction of the fluidization gas through the fluidized bed
reactor; and
The one or more outlets (9) in the upper zone (7) of fluidized bed
reactor (1) are connected with one or more outlet lines (33) for
conveying the fluidization gas stream (34) to gas/solid separation
means (2). The gas/solids separation means (2) comprise two
outlets, one top outlet (27) and one bottom outlet (28). The
fluidization gas exits the top outlet (27) as overhead stream
(42).
The overhead stream (42) is conducted via gas circulation line (38)
to the inlet (8) in the bottom zone (5) of the fluidized bed
reactor (1) whereby a fluidization gas circuit is established. The
gas circulation line (38) includes means for pressurizing (4) and
means for cooling (3) of the gas. Optionally, the gas circulation
line (38) further includes solid filter means (41) for further
reducing the amount of solids and especially fines still contained
in the fluidization gas stream. These solid filter means (41) are
located upstream from the means for pressurizing (4) and means for
cooling (3).
The solid recycling stream (36) mainly containing solids exits the
gas/solids separation means (2) by the bottom outlet (28). The
solid recycling stream (36) is conveyed via solid recycling line
(35) to the solid recycling inlet (37) in the middle zone (6) of
the fluidized bed reactor (1) whereby a solid circuit is
established. The solid recycling line (35) includes a flow through
device (29). The flow though device (29) varies the solid recycling
stream (36), whereby the whole solid recycling stream (36) or only
a part of it is delivered to the fluidized bed reactor (1).
Moreover, if the operating conditions in the fluidized gas reactor
are such that more or less no fines are produced, the flow through
device (29) may be fully closed such that there is no recycling
taking place.
Furthermore, by the flow through device (29) a stream to downstream
process stages (40) can branched off from the solid recycling
stream (36). Therefore, the flow through device (29) can also vary
the stream to downstream process stages (40) and/or an outlet. The
stream to downstream process stages (40) corresponds to the part of
the solid recycling stream (36) not being delivered to the
fluidized bed reactor (1).
Furthermore, in the fluidization gas stream (34) at the outlet (9)
of the fluidized bed reactor (1) the mean particle size and/or the
particle size distribution is measured. Additionally, the operation
condition inside the fluidized bed reactor (1) are analyzed.
Analysis typically includes the measurement of the fluidization
velocity, u.sub.f, and/or comonomer concentration, C.sub.c.
The obtained data (30) regarding particle size and operation
condition are sent to a controller (31). The data (30) are
processed by a controller (31). The controller (31) and the flow
through device (29) communicate with each other by sending one or
more signals in at least one direction between the flow through
device (29) and the controller (31). Due to that communication the
flow through device (29) is adjusted by the controller (31),
whereby the flow though device (29) varies in the following at
least the solid recycling stream (36) back to the fluidized bed
reactor (1) as outlined above.
The reactor assembly according to FIG. 4 is a modification of the
reactor assembly according to FIG. 1.
The gas/solid separation means (2) shown by the dotted rectangle
contain a first gas/solid separation unit (43) and a second
gas/solid separation unit (44). The first gas/solid separation unit
(43) has a top outlet (48) for the unit overhead stream of first
gas/solid separation unit (47). The unit overhead stream of first
gas/solid separation unit (47) is fed to the second gas/solid
separation unit (44). The unit overhead stream of the second
gas/solid separation unit obtained through outlet (27) is the
overhead stream (42).
The unit solid recycling streams (46) obtained through outlets (45)
are combined. These combined streams are the solid recycling stream
(36) obtained in solid recycling line (35).
EXAMPLES
Example 1 (Comparative)
The invention was exemplified with a reactor made of steel having
the following dimensions:
Height of the bottom zone: 1680 mm
Diameter at the bottom of the bottom zone: 175 mm
Height of the middle zone: 2050 mm
Height of the upper zone: 670 mm
Diameter of the middle zone: 770 mm
The operation of the reactor was stable and without problems.
The reactor described above was used for copolymerization of
ethylene and 1-butene at a temperature of 80.degree. C. and a
pressure of 20 bar. The height of the fluidized bed, calculated
from the bottom of the middle zone was 2100 mm.
Ethylene homopolymer (MFR.sub.2=300 g/10 min, density 974
kg/m.sup.3) produced in a loop reactor and still containing the
active catalyst which one dispersed therein was introduced into the
above reactor via an inlet located in the bottom zone at a rate of
40 kg/h. Ethylene, hydrogen and 1-butene were continuously
introduced into the circulation gas line so that the ethylene
concentration in the fluidization gas was 17% by mole, the ratio of
1-butene to ethylene was 100 mol/kmol and the ratio of hydrogen to
ethylene was 15 mol/kmol. The reminder of the fluidization gas was
nitrogen. The flow rate of the gas was adjusted so that the
superficial gas velocity in the middle zone of the reactor was 15
cm/s. The resulting copolymer could be easily withdrawn via an
outlet at a rate of 68 kg/h.
The fluidization gas flow was thus 250 m.sup.3/h, corresponding to
a mass flow rate of 4800 kg/h. The polymer content in the
fluidization gas stream withdrawn from the top of the fluidized bed
reactor was about 0.25% by weight and thus 12.0 kg/h of polymer was
withdrawn together with the fluidization gas. The fluidization gas
stream was passed through a cyclone where 11.4 kg/h of the polymer
was recovered from the cyclone as a bottom stream while 0.6 kg/h
remained with the fluidization gas stream. The entire bottom stream
from the cyclone was combined with the product stream withdrawn
from the reactor and directed to product recovery where it was
mixed with additives and extruded to pellets. The solid recycling
stream branched off to the output of the polymer downstream from
the flow through device was combined for product recovery with the
polymer stream withdrawn from the fluidized bed reactor via outlet
of the polymer.
Example 2 (Inventive)
Procedure of Example 1 was repeated. Then, the solid recycling
stream withdrawn at the bottom outlet from the cyclone was returned
to the fluidized bed reactor. When the reactor was operated in this
way the bed level increased in one hour to 2300 mm. At the same
time the solids content in the fluidization gas stream increased to
0.4% by weight and 19.2 kg/h of polymer were transported by the
fluidization gas stream to the cyclone. The flow of polymer
captured from the solid recycling stream at the bottom outlet of
the cyclone was 18.9 kg/h and 0.3 kg/h remained in the in the
overhead stream of the cyclone. The polymer withdrawal rate from
the bed was then lowered to 61 kg/h. After one hour of operation
the flows were changed so that the solid recycling stream (36) was
11.5 kg/h, the stream to downstream process stages (40) was 7.5
kg/h and polymer withdrawal stream (14) from the reactor was 72
kg/h. A stable operation was achieved with no problem of fouling in
the circulation gas system.
In the Following Clauses Preferred Embodiments of the Invention are
Described
1. A reactor assembly for the production of polymers including a
fluidized bed reactor (1) comprising a bottom zone (5), a middle
zone (6) and an upper zone (7), one or more outlets (9) for
fluidization gas streams (34) located in the upper zone (7),
gas/solid separation means (2), a flow through device (29), a solid
recycling line (35), a solid recycling inlet (37), a gas
circulation line (38), an inlet (8) for fluidization gas located in
the bottom zone (5); the outlet (9) for the fluidization gas stream
(34) being coupled with the fluidized bed reactor (1) via gas/solid
separation means (2), gas circulation line (38) and inlet (8) and
via solid recycling line (35), gas/solid separation means (2) and
solid recycling inlet (37); the equivalent cross-sectional diameter
of the bottom zone (5) being monotonically increasing with respect
to the flow direction of the fluidization gas through the fluidized
bed reactor (1); and wherein there is an unobstructed passageway in
the direction of flow of the fluidization gas through the fluidized
bed reactor from the bottom zone (5) to the upper zone (7),
characterized in that the solid recycling line (35) includes the
flow through device (29). 2. The reactor assembly according to any
one of the preceeding clauses, wherein said fluidized bed reactor
(1) comprises no gas distribution grid and/or plates. 3. The
reactor assembly according to any one of the preceeding clauses 2
to 10, wherein the gas solids/separation means (2) are cyclones. 4.
Method for operating a reactor assembly for the production of
polymers including a fluidized bed reactor (1) comprising a bottom
zone (5), a middle zone (6) and an upper zone (7), one or more
outlets (9) for fluidization gas streams (34) located in the upper
zone (7), gas/solid separation means (2), a flow through device
(29), a solid recycling line (35), a solid recycling inlet (37), a
gas circulation line (38), an inlet (8) for fluidization gas
located in the bottom zone (5) and a controller (31), the method
comprising the steps of: a) measuring the mean particle size and/or
the particle size distribution and/or the concentration of all
solids of an fluidization gas stream (34) from the fluidized bed
reactor (1); b) analyzing the operation conditions in the fluidized
bed reactor (1); c) sending the data (30) obtained in steps a) and
b) to a controller (31); d) processing the data (30) by the
controller (31); and e) adjusting the flow through device (29) by
the controller (31); whereby the flow through device (29) varies
the solid recycling stream (36) via solid recycling line (35) back
to the fluidized bed reactor (1). 5. Method according to clause 4,
wherein step b) comprises the measurement of fluidization velocity,
u.sub.f. 6. Method according to clause 4 or 5, furthermore
comprising the steps of: dd) predicting the mean particle size
and/or the particle size distribution of the fluidization gas
stream (34) using the data obtained in step b); de) comparing the
measured and the predicted mean particle size and/or the particle
size distribution of the fluidization gas stream (34) of steps a)
and dd). 7. Method according to any of the preceding clauses 4 to
6, whereby the solid recycling stream (36) is increased when the
content of fines deduced from the measured particle size
distribution of step a) is larger than a predetermined set point
for the content of fines. 8. Method according to one of the
preceding clauses 4 to 7, wherein the flow through device (29)
varies the solid recycling stream (36) back to the fluidized bed
reactor (1) and/or allows a stream to downstream process stages
(40). 9. Method according to one of the preceding clauses 4 to 8
wherein said fluidization gas is upwards rising fluidization gas
and said upwards rising fluidization gas has a superficial velocity
in the middle zone (6) of from 0.05 to 0.8 m/s. 10. A method for
polymerizing olefins in a fluidized bed reactor (1), wherein the
fluidized bed is formed by polymer particles in an upwards rising
fluidization gas said upwards rising fluidization gas has a
superficial velocity in the middle zone (6) of from 0.05 to 0.8
m/s, said method comprising the steps of: (i) withdrawing a
fluidization gas stream (34) via outlet (9) from said fluidized bed
reactor (1) at a height of more than 90% of the total height of
said fluidized bed reactor (1); (ii) separating polymer particles
from said fluidization gas stream (34) to produce an overhead
stream (42) and a solid recycling stream (36); (iii) branching off
from said solid recycling stream (36) a stream to downstream
process stages (40); (iv) directing said stream to downstream
process stages (40) to downstream process stages; and (v) recycling
the solid recycling stream (36) into said fluidized bed reactor
(1); wherein said fluidized bed reactor (1) comprises a bottom zone
(5), a middle zone (6) and an upper zone (7), the equivalent
cross-sectional diameter of the bottom zone (5) being monotonically
increasing with respect to the flow direction of the fluidization
gas through the fluidized bed reactor and the equivalent
cross-sectional diameter of the upper zone (7) being monotonically
decreasing with respect to the flow direction of the fluidization
gas through the fluidized bed reactor (1); and wherein there is an
unobstructed passageway in the direction of flow of the
fluidization gas through the fluidized bed reactor from the bottom
zone (5) to the upper zone (7). 11. The method according clause 10,
wherein the fluidized bed reactor (1) further comprises one or more
outlets (9) for fluidization gas streams (34) located in the upper
zone (7). 12. The method according to any one of the preceeding
clauses 4 to 11, wherein said fluidized bed reactor (1) comprises
no gas distribution grid and/or plates. 13. The method according to
any one of the preceeding clauses 4 to 12, wherein the gas
solids/separation means (2) are cyclones. 14. Use of a controller
(31), a flow through device (29) and a solid recycling line (35) in
a reactor assembly for the production of polymers including a
fluidized bed reactor (1) comprising a bottom zone (5), a middle
zone (6) and an upper zone (7), one or more outlets (9) for
fluidization gas streams (34) located in the upper zone (7),
gas/solid separation means (2), a solid recycling line (35), a
solid recycling inlet (37), a gas circulation line (38), an inlet
(8) for fluidization gas located in the bottom zone (5) and an
outlet for the polymer (14) for minimizing the mass fraction of
fines with respect to the solids contained in the fluidization gas
streams (34). 15. Use of a controller (31), a flow through device
(29) and a solid recycling line (35) according to clause 14 in a
reactor assembly for the production of polymers for minimizing the
amount of fines produced by the fluidized bed reactor (1), wherein
the minimizing occurs by regulating the amount of fines produced in
the fluidized bed reactor (1) by varying the solid recycling stream
(36) and/or the operation conditions of the fluidized bed reactor
(1).
* * * * *